Dynamic range control system

ABSTRACT

An automatic dynamic range control system for eliminating dynamic gain limitations that may provide undesirable effects in the moving target indicator or video processor portions of a radar system. A separate auxiliary measurement channel is provided in parallel with the main IF channel having a logarithmic amplifier or sequential logarithmic detector with sufficient dynamic range to meet the signal requirements. At each range bin in the auxiliary channel, the IF signal is quantized in amplitude above the predetermined dynamic range of the main channel by selecting relatively large input signal level ranges which, in the system, provides a relatively low quantizing accuracy without substantially decreasing the system reliability. The quantized levels are then passed through a switch which selects the last PRF interval used for a fill pulse (such as in an MTI system) and places the quantized signals into a 1/PRF shift register. The quantized levels are then used in subsequent sweeps of the dwell period during each range bin to attenuate signals in the main IF channel that would be limited by gain characteristics of that channel. By utilizing predetermined gain control quantized values for an entire dwell period, undesired amplitude modulations that may affect the moving target indicator or processing operation are eliminated.

United States Patent [191 Effinger et al.

[ Jan. 15, 1974 DYNAMIC RANGE CONTROL SYSTEM [75] inventors: David D.Effinger, La Habra; Norol T. Evans, San Pedro; Vaughn H. Estrick,Fullerton, all of Calif.

[73] Assignee: Hughes Aircraft Company, Culver City, Calif.

[22]. Filed: Mar. 10, 1972 [2111] Appl. No.: 233,834

Primary ExaminerBenjamin A. Borchelt Assistant Examiner-G. E. MontoneAttorney-W. H. MacAllister et al.

[57] ABSTRACT An automatic dynamic range control system for elimi- 343/7A, 343/5 cc nating dynamic gain limitations that may provide undesirableetfects in the moving target indicator or video processor portions of aradar system. A separate auxiliary measurement channel is provided inparallel with the main IF channel having a logarithmic amplifier orsequential logarithmic detector with sufficient dynamic range to meetthe signal requirements. At each range bin in the auxiliary channel, theIF signal is quantized in amplitude above the predetermined dynamicrange of the main channel by selecting rela tively large input signallevel ranges which, in the system, provides a relatively low quantizingaccuracy without substantially decreasing the system reliability. Thequantized levels are then passed through a switch which selects the lastPRF interval used for a fill pulse (such as in an MTl system) and placesthe quantized signals into a l/PRF shift register. The quantized levelsare then used in subsequent sweeps of the dwell period during each rangebin to attenuate signals in the main lF channel that would be limited bygain characteristics of that channel. By utilizing predetermined gaincontrol quantized values for an entire dwell period, undesired amplitudemodulations that may affect the moving target indicator or processingoperation are eliminated.

12 Claims, 4 Drawing Figures Timing Control PRF Shift :20 I22} Reg I32Gate Video Utilization Proc Units DYNAMIC RANGE CONTROL SYSTEMBACKGROUND OF THE INVENTION 1. Field of the Invention namic rangelimitations of a radar channel, without inl troducing undesiredamplitude modulations.

2. Description of the Prior Art Radar systems, utilizing digital MTI ordoppler processing characteristically have dynamic range limita- 1 tionsin the main IF channel such as in the analog to digital converter whichclips the signal, thus causing a loss of target or tracking information.If the signal is limited in the radar channel, the possible combining ofclutter energy signals may cause erroneous detection of clutter energyas target energy. Another problem with limiting in the radarchanncl isthat amplitude information may be lost that is required for accuratetracking and for reliable beam splitting. If a conventionalinstantaneous automatic gain control system is utilized, the amplitudemodulation on the top of the signal is substantially eliminated,resulting in the inability to reliably detect targets. Thus, in radarsystems, and especially, in digital processor type radar systems, thelimited dynamic range of the IF channel results in a considerable degreeof inaccurate and unreliable operation.

It is therefore an object of this invention to provide an improved radarsystem that substantially eliminates the undesired effects of limiteddynamic ranges of operation in the radar channels.

It is another object of this invention to provide an improved andreliable digital moving target indicator systern.

It is another object of this invention to provide a large dynamic rangeradar system which operates by retaining a maximum of signalinformation.

It is a further object of this invention to provide a gain control unitfor radar system channel which operates to substantially eliminatelimiting or clipping of the radar signal.

BRIEF DESCRIPTION OF THE DRAWINGS These and other objects, features andadvantages of the invention itself will become apparent to those skilledin the art, in the light of the following detailed description, taken inconsideration with the accompanying drawings wherein like referencenumerals indicate like or corresponding parts throughout the severalarts: p FIG. I is a block diagram of the improved dynamic rangeeontrolsystem in accordance with the invention;

FIG. 2 is a schematic block diagram of an example of an analog todigital encoder or analog to digital converter that maybe utilized inthe system of FIGS. la and 1b.

FIG. 3 is a schematic diagram of waveforms of voltage as a function oftime for explaining the operations of the system of FIGS. 1a and lb; and

FIG. 4 is a schematic diagram of waveforms showing dynamic range as afunction of time for further explaining theattenuation operation of thesystem of FIGS. la and lb.

DETAILED DESCRIPTION OF THE INVENTION The system of the inventionincludes an auxiliary channel 10 utilized to control the gain in a mainchannel 12 illustrated as an IF channel but which in some arrangementsof the invention may be at other frequencies, such as at RF (radiofrequency). The radar system includes an antenna 14 which may be aplanar array or a parabolic dish, for example, and an antenna control 0circuit 16 which may include microwave connections and scan controlunits coupled to a suitable duplexer 18 which, as is well known in theart, is then coupled to a transmitter 20 and a receiver 22. Pulses ofenergy are transmitted into space from the transmitter 20 in 5 responseto a timing control circuit 26 and intercepted energy is then applied tothe receiver 22 and converted to IF (intermediate frequency) signals ina conventional manner and applied through a lead 30 to a suitable powersplitter 32. The IF signal on the lead 30 may, for example in theillustrated system, have a dynamic range of 74 db and the channel 12 mayhave a limited dynamic range of, for example, 50 db. Dynamic range isdefined as the ratio of the specified maximum signal level capability ofthe system or components to its noise level. Each signal has dynamicrange requirements for providing a desired amount of amplitudeinformation. Thus, dynamic range which may be expressed in decibels, isa measure of the value of the S/N (signal to noise) ratio over which asystem or component can operate. The IF signal is then applied from thepower splitter 32 through a lead 36 to an attenuator unit 38 which mayinclude attenuators 40, 42, and 44 coupled in series and respectivelyproviding an attenuation ofO or 6 db, ofO or 12 db, and of() or 18 1b.The attenuator 40, for example, includes a power splitter 48 coupled tothe lead 36 to apply energy through a 6 db attenuator 50 when a switch54 is conductive in a first position, or through a lead 52 and theswitch 54 to an output lead 56 when the switch is in a second position.The lead 56 then selectively passes the signal through a similarattenuator element of the attenuator 42 which in turn passes the signalsthrough a lead 58 to a similar attenuator element depending on a switchposition of the attenuator 44.

The signal is then applied through a suitable lead 60 to a phasedetector 62 and in turn to an MTI unit 64, which may be either aconventional moving target indicator (MTI) unit as shown, or may be anin phase and in quadrature target indicator unit. The phase detector 62receives a local oscillator (L.O.) reference signal from the receiver22, as is well known in the art. If quadrature MTI is utilized, thephase detector 62 is a synchronous phase detector, as is well known inthe art, providing detection at two phases 90 separated from each other.In a quadrature system the phase detector has two outputs (I & Q) andtherefore two parallel moving target indicator systems would be used, asis well known in the art. The illustrated MTI unit 64 includes ananalog-to-digital converter responsive to the video signal on a lead 72as provided by the phase detector 62. A first stage of the MTI unit 64includes a shift register 74 responsive on a lead 76 to the A to Dconverter 70 applying a signal, range bin by range bin through a lead 78to a first subtractor 80, also receiving the input signal on the lead76. As is well known in the art, the shift register may have a totaldelay of a range sweep providing comparisons and applying a res iduefrom the subtractor 80 to a lead 84 and, in turn, to a second shiftregister 86. The signal delayed in the shift register 86 after a rangesweep delay is applied on a range bin to range bin basis to a subtractor99, in combination with the undelayed signal on the lead 84 to provide asecond reside along with the target signal on a lead 100 which isapplied to a video processor 102. The video signal after processing thenmay be applied through a lead 104 to a suitable utilization unit 106which may, for example, include displays and aircraft control circuitry,as are well known in the art.

The main IF channel 12 may have a substantial limitation of dynamicrage, which is illustrated as 50-db and may be caused, for example, bythe A to D converter 70, the phase detector 62 and other amplifiers thatmay be required in the channel. The auxiliary channel includes alogarithmic amplifier or sequential logarithmic detector 110. As is wellknown in the art, logarithmic amplifiers or detectors use successivedetection over predetermined ranges of gain to provide a wide dynamicrange. A sequential detector that may be utilized in the system of theinvention is shown, for example, on pages 5-34 of the book RadarHandbqok, by Merril I. Skolnik, copyright 1970 and published by McGrawHill Book Company. The logarithmic amplifier 110, which is coupled fromthe power splitter 32 by a lead 112, has at least the required dynamicrange of the IF signal which, for example, may be 7 f1 db .T heamplified ignal is applied from th e log amplifier 110 through a lead116 to an analog-to-dig iTl converter 118 which detects the amplitude ofthe signal e a AT a id generates 9 ense s br slzi quantized outputdigital signals on a composite lead 120, which signals are applied to agate or switch 122. It is to be noted that the reference level is takenat 47 db rather than 50 db because the system of the invention allows alow level tolerance of selection of the dynamic range correction. Inorder to reduce the accuracy requirements of the quantization circuitsof the A to D converter 118, the following levels are selected:

Input Signal Level Ouantized Output Attenuation s 47db 000 0 db 47db s s53db 001 6 db 53db s s 59db 010 12 db 59db s s 65db 011 I8 db 65db s sI00 24 db As can be seen by the above table, each input signal rangeabove 47 db has a corresponding attenuation that will be inserted in themain IF channel 12. The gate 122, which in response to the timingcontrol circuit 26 passes the encoded data of the last PRF interval usedfor a fill pulse, into a shift register unit 130 or l/PRF register whichhas a delay of one radar sweep.

During the process time as determined by the timing control circuit 26the contents of the shift register 130 are applied range bin by rangebin through a gate 132 and, in turn, through a composite lead 134 to adecode logic circuit 136. In response to the three bit code on the lead134, the logic decode circuit 136 energizes different combinations ofthe attenuators 40, 42, and 44 to provide the required degree ofattenuation to the IF signal on the lead 36. A clock 140 may be providedto control the digital operation throughout the system and apply clocksignals to a divide circuit 142, which in turn applies signals to thetiming control 26 for controlling the time of transmission, the gatingof the gate 122 and the processing time.

The A to D converter 1 18 may be of any suitable type responsive to anumber of voltage ranges such as illustrated in FIG. 2, in which theanalog input voltage on the lead 116 is applied to a subtractor alsoreceiving a reference voltage of 5.2 volts. In the illustratedarrangement it is assumed that the log scale factor is 10 db per volt,so that 0 db is equal to 0.5 volts, 10 db is equal to 1.5 volts, and 20db is equal to 2.5 volts. As a result, the quantized outputs aredetermined from the following voltage ranges:

The signal at the output of the subtractor 150 is then applied to thecompare circuit 152 to determine if the output is positive or negative,and if it is negative, the

7 signal is applied on a lead 154 representing 000 to a decode circuit156. If the output of the circuit 150 is positive, an AND gate 158responds to the positive condi- 1 of the circuit 160 is positive, an ANDgate 168 applies the voltage to a subtractor circuit 172 which againsubtracts 0.5 volts. If the remainder from the circuit 172 is negativeas determined by a compare circuit 174, the signal representative of 010condition is applied to a lead 176 to the decode circuit 156. If theremainder is positive, an AND gate 180 applies the remainder to asubtractor circuit 182 which again subtracts 0.5 volts therefrom. If theoutput from the circuit 182 is negative, a compare circuit 184 applies asignal representative of a 011 condition to a lead 186 which is in turnapplied to the decode circuit 156. If the signal developed by thesubtractor 182 is positive, an AND gate 190 applies the voltage to asubtractor 192, which again subtracts 0.5 volts from the signal valueand applies a signal to a compare circuit 196. If the difference isnegative, the compare circuit 196 applies a signal on a lead 198representative of a 100 condition to the decode circuit 156. A clockpulse from the clock 140 may be utilized with a suitable delay to gatethe detected results during each range bin through the decode circuit156.

The decode circuit 156 responds to the true or false conditions of eachof the leads 154, 166, 176, 186, and 198 to set flip-flops 202, 204, and206 of respective shift registers 208, 209, and 210 of the shiftregister unit 130. Thus, the subtraction and comparison operation of thecircuit of FIG. 2 may be timed in response to clock pulses to rapidlyset the flip-flops 202, 204, and 206 during each range bin period. Inother arrangements in accordance with the invention, several range binsdelays may be provided in the A to D converter 118.

Referring now principally to FIG. 1, the decode logic unit 136 respondsto the input code by controlling the attenuators 40 42, and 44,respectively represented by 13,321 C in accordance WitI Ee followingtable:

loo

Thus, it can be seen that each required attenuation condition isprovided by the three attenuators of the attenuation unit 38.

Referring now also to the waveforms of FIG. 3, the transmit pulses ofthe waveform 220 may be provided by the transmitter as continuous pulseswhich repetitively may have a sequence of four fill pulses, three MTIpulses for the illustrated two-stage canceller, 8 pulses for makingdoppler determination, followed by the same sequence. In somearrangements within the scope of the invention, different transmitperiods or sequences of fill pulses may be at different pulserepetitionfrequencies. As is well known in the art, when utilizing anMTI system or a doppler filter system, four fill terso the proper andtotal cancellation is provided. During the last fill pulse interval, apulse of a waveform 222 may be developed by a continuous repetitive sequence in the timing control circuit 26 to close the gate 122, so thatthe sampled amplitude code from the A to Diconverter 118 is applied tothe shift register 130 during that entire range sweep period in a rangebin by range bin fashion over the entire range of operation of thesystem; After the shift register 130 is filled and at the beginning ofthe next transmit pulse, a process time pulse of a waveform 226 isprovided by a fixed repetitive sequence in the timing control circuit 26to close the gate 132 to provide the attenuation of the attenuator, unit38 as a function of the excess dynamic amplitude sampled during therange sweep after the fourth transmitted pulse. It should be noted thatif new data were used in each PRF interval instead of using one PRFinterval of data to control the entire dwell period, moving targets maybe degraded or eliminated.

For example, if a moving target at optimum radial velocity causes a 6 dbmodulation on the clutter input signal amplitude on a pulse-to-pulsebasis, the signal plus clutter is larger than +50 db above RMS noise,and an instant automatic gain control circuit would eliminate this AMmodulation and substantially eliminate the target. Thus, the use of a l/PFR memory circuit in the systemin accordance with the inventionprovides a substantial detection and tracking advantage. For acombination moving target indicator and range gated pulse dopplerprocessor, the register is loaded once and its contents are then used tocontrol the gain for the remainder, of the dwell period in accordancewith the invention. It is to be understood that the invention is notlimited to any particular number of fill pulses, but is applicablewherever a specific pulse may be utilized during an operation period toprovide highly reliable MTI or doppler operation.

Referring now principally to l lfii. 4 as well as to FIGS. 1 and 3,signals 230, 231, and 232 show the amplitudeof the IF signal duringrange sweeps l to 15, as indicated in FIG. 3.:3Dun'ng the range sweep ofthe last fill pulse,a signal of the waveform 230 shows the IF signal.amplitude for rangebin l to range bin n, with n being the total numberof range bins utilized in the sys- [6111s During approximately the firstfour range bins between times t. and t,, the IF signal of the waveform230 exceeds 47 db in amplitude and coded signals are stored in the shiftregister 130 representing that amplitudeli range. During the next groupof range bins between times t, and t the amplitude of the IF signal ofthe last fill pulse is below the 47 db level so that no atpulses areprovided to obtain a total sample ofthe clut-fi tenuation is requiredand a series of 000 values are stored in the shift register 130. Duringthe range bins between times and t the signal has an amplitude greaterthan 47 dbs so that coded attenuation values are stored in the shiftregister 130. During the range bins between times t; and 2: of the lastfill pulse range sweep, the signal level is below the 47 db level and000 codes are stored in the shift register 130, indicating that anattenuation of the radar signal is not required.

During the next range sweep, which is range sweep 5, the coded signalsare applied during each range bin to the decode logic circuit 136 sothat during the range bin between times t and t, the signal i attenuatedbelow the 47 db level, during the range bins i, to 22 the signal isunchanged, between the range bins between times 1 and tg the signal isattenuated below the 4711b level, and between the time range bins oftimes to t, the signal is unattenuated. Thus, it can be seen that theattenuation only has an effect when the signal as sensed in theauxiliary channel 10 exceeds the dynamic range limitation of the mainchannel 12. During the subsequent sweeps of the dwell period, sweeps 6to 15, the signal is controlled in a similar manner as indicated by thewaveform 232. Thus, the system utilizes the sampled dynamic range forthe subsequent range sweeps of the dwell period, which is the periodthat the same area in space is illuminated, so that undesired amplitudemodulations are not provided.

It is to be understood that the illustrated dynamic range of the main IFchannel and the auxiliary channels has only been selected forexplanatory purposes and the invention is not to be limited to anyparticular dynamic ranges, but the principles are applicable to anysystem in which some limitation of the dynamic range may be provided inthe main channel.

Thus there has been provided an automatic dynamic range control systemto provide gain control in a digital MTI or doppler system to preventeither limiting in the IF circuits or in the RF circuits, such as mayoccur in the A to D converters. An automatic gain control sys tem isprovided that does not eliminate the desired sig nal modulations as aresult of storing the amplitude during a selected pulse of the operatingcycle. By applying the same gain control function on each PRF intervalof a plurality of intervals of the dwell period, AM modulation isprevented from being introduced into the MTI residue. The use of anauxiliary large dynamic range logarithmic channel for measurementpurposes allows the gain control function to be applied to the exactrange that is required in the main channel, instead of some finite timeafter the large signal, as in the case of a conventional automatic gaincontrol system. The system has been found to operate reliably byutilizing a relatively large increment of quantizing accuracy. Thesystem is equally applicable to MTI or other radar systems, such as amonopulse tracking system.

What is claimed is:

l. A dynamic range control system responsive in a radar system to aninput radar signal having a predetermined dynamic range comprising afirst channel responsive to said input radar signal and having a limiteddynamic range less than said predetermined dynamic range,

attenuation means included in said first channel,

a second channel responsive to said input radar signal, said secondchannel being characterized by the absence of attenuation means andhaving a dynamic range of operation to include the predetermined dynamicrange of said input radar signal,

first means in said second channel for sampling and storing theamplitude characteristics of said input radar signal which is notattenuated during a selected range sweep, and

second means coupled to said first means in said second channel and tosaid attenuation means in said first channel for controlling saidattenuation means to selectively attenuate the input radar signal to lwhich said first channel is responsive only as afunction of theamplitude characteristics of the input radar signal stored in the firstmeans of said second channel, and for inhibiting said attenuation meansfrom attenuating said input radar signal in said first channel when theamplitude characteristics of said input radar signal are indicative ofan input radar signal which is substantially within the limited dynamicrange of said first channel.

2. The system of claim 1 in which the first means for sampling stores acode representative of the signal dynamic range substantially above thedynamic range of said first channel so that the signal in said firstchannel is only attenuated if its dynamic range is above the limiteddynamic range thereof.

3. The system of claim 2 in which said second channel includeslogarithmic means for providing an output which is a logarithmicfunction of the input radar signal applied to said second channel.

4. The system of claim 3 in which said first means includes an encoderand a shift register and in which timing means is provided so that acode is stored in said shift register during a selected range sweep forcontrolling said attenuation means.

5. The system of claim 4 in which gating means is included in said firstmeans responsive to said timing means so that during a first range sweepperiod the code is stored and said attenuation means is controlled fromthe stored code during a predetermined number of subsequent rangesweeps.

6. The system of claim 5 in which said radar system has a repetitivesequence of a selected number of fill pulses and a selected number ofoperation pulses and in which said switching means includes a first gatecoupled between said encoding means and said shift register andcontrolled by said timing means in response to a range sweep defined bya selected fill pulse and a second gate coupled between said shiftregister and said attenuation means for applying the stored code on arange bin basis to said attenuation means during range sweeps defined bythe subsequent operation pulses.

7. In a radar system having a repetitive sequence of transmitting aselected number of fill pulses following by a selected number ofoperational pulses each defining a range sweep and having a sequence ofrange bin periods during each range sweep, and in which said radarsystem includes a radar channel including attenuation means, said radarchannel being responsive to an input radar signal and having a limiteddynamic range, a dynamic range control system for controlling theattenuation of input radar signals in said radar channel comprisinglogarithmic amplifier means responsive to said radar signal,

an analog-to-digital converter coupled to said logarithmic amplifiermeans for developing coded signals during each range bin period,

storage means coupled to said analog-to-digital converter means forstoring said coded signals,

attenuation control means coupled'to said attenuation means in saidradar channel and to said storage means for controlling said attenuationmeans to attenuate the input radar signal in said radar channel as afunction of the coded signals stored in said storage means, and

control means coupled to said storage means for providing control tostore said coded signals during a range sweep defined by the last fillpulse and apply ing said stored coded signals to said attenuation meansduring each range sweep defined by the following operational pulses.

8. The system of claim 7 in which said analog-todigital converterdevelops a coded signal of predetermined values during each range binperiod and in which said attenuation control means includes decodingmeans responsive to said stored coded signal to attenuate said radarsignal during range bin periods when said radar signal is limited by thedynamic range of said radar channel.

9. The system of claim 8 in which said storage means includes a firstgate between said analog-to-digital converter means and said storagemeans for transferring the coded signals to said shift register duringthe range sweeps defined by the last fill pulses, in which said storagemeans is a recirculating shift register and in which said storage meansalso includes a second gate coupled between said shift register and saiddecoding means for applying signals representative of the stored codesduring the range sweeps defined by the operational pulses.

10. A radar system having transmitting and receiving means comprising afirst channel coupled to said receiving means for receiving an [Fsignal,

attenuation means coupled in said first channel,

moving target indicator means coupled in said first channel,

a second channel coupled to said receiving means for receiving said IFsignal,

sequential detector means included in said second channel, encodingmeans included in said second channel and coupled to said sequentialdetector means, and

storage means included in said second channel and coupled to saidencoding means for storing coded signals during a selected range sweepand coupled to said attenuation means for controlling the IF signal insaid first channel during a selected plurality of range sweeps.

l 1. In a radar system of the type including a radar receiver channelresponsive to an input radar signal having a predetermined dynamicrange, said radar receiver channel being characterized by a limiteddynamic range, smaller than the predetermined dynamic range of saidinput radar signal, an arrangement comprising logarithmic meansresponsive to said input radar signal for providing an output signalwhich is a function of the unattenuated input radar signal, saidlogarithmic means having a dynamic range including said predetermineddynamic range,

first means coupled to said logarithmic means for storing any one of aplurality of codes as a function of the amplitude of said output signal,

signal is below a preselected level and for storing N other codes whenthe output signal level is in any one of N different level intervalsabove said preselected level, and said second means inhibit saidattenuation means from attenuating said input radar signal in saidreceiver channel in response to said first code, and con trol saidattenuation means to attenuate tne input radar signal in said receiverchannel by fixed different factors in response to said N codes,respectively.

1. A dynamic range control system responsive in a radar system to aninput radar signal having a predetermined dynamic range comprising afirst channel responsive to said input radar signal and having a limiteddynamic range less than said predetermined dynamic range, attenuationmeans included in said first channel, a second channel responsive tosaid input radar signal, said second channel being characterized by theabsence of attenuation means and having a dynamic range of operation toinclude the predetermined dynamic range of said input radar signal,first means in said second channel for sampling and storing theamplitude characteristics of said input radar signal which is notattenuated during a selected range sweep, and second means coupled tosaid first means in said second channel and to said attenuation means insaid first channel for controlling said attenuation means to selectivelyattenuate the input radar signal to which said first channel isresponsive only as a function of the amplitude characteristics of theinput radar signal stored in the first means of said second channel, andfor inhibiting said attenuation means from attenuating said input radarsignal in said first channel when the amplitude characteristics of saidinput radar signal are indicative of an input radar signal which issubstantially within thE limited dynamic range of said first channel. 2.The system of claim 1 in which the first means for sampling stores acode representative of the signal dynamic range substantially above thedynamic range of said first channel so that the signal in said firstchannel is only attenuated if its dynamic range is above the limiteddynamic range thereof.
 3. The system of claim 2 in which said secondchannel includes logarithmic means for providing an output which is alogarithmic function of the input radar signal applied to said secondchannel.
 4. The system of claim 3 in which said first means includes anencoder and a shift register and in which timing means is provided sothat a code is stored in said shift register during a selected rangesweep for controlling said attenuation means.
 5. The system of claim 4in which gating means is included in said first means responsive to saidtiming means so that during a first range sweep period the code isstored and said attenuation means is controlled from the stored codeduring a predetermined number of subsequent range sweeps.
 6. The systemof claim 5 in which said radar system has a repetitive sequence of aselected number of fill pulses and a selected number of operation pulsesand in which said switching means includes a first gate coupled betweensaid encoding means and said shift register and controlled by saidtiming means in response to a range sweep defined by a selected fillpulse and a second gate coupled between said shift register and saidattenuation means for applying the stored code on a range bin basis tosaid attenuation means during range sweeps defined by the subsequentoperation pulses.
 7. In a radar system having a repetitive sequence oftransmitting a selected number of fill pulses following by a selectednumber of operational pulses each defining a range sweep and having asequence of range bin periods during each range sweep, and in which saidradar system includes a radar channel including attenuation means, saidradar channel being responsive to an input radar signal and having alimited dynamic range, a dynamic range control system for controllingthe attenuation of input radar signals in said radar channel comprisinglogarithmic amplifier means responsive to said radar signal, ananalog-to-digital converter coupled to said logarithmic amplifier meansfor developing coded signals during each range bin period, storage meanscoupled to said analog-to-digital converter means for storing said codedsignals, attenuation control means coupled to said attenuation means insaid radar channel and to said storage means for controlling saidattenuation means to attenuate the input radar signal in said radarchannel as a function of the coded signals stored in said storage means,and control means coupled to said storage means for providing control tostore said coded signals during a range sweep defined by the last fillpulse and applying said stored coded signals to said attenuation meansduring each range sweep defined by the following operational pulses. 8.The system of claim 7 in which said analog-to-digital converter developsa coded signal of predetermined values during each range bin period andin which said attenuation control means includes decoding meansresponsive to said stored coded signal to attenuate said radar signalduring range bin periods when said radar signal is limited by thedynamic range of said radar channel.
 9. The system of claim 8 in whichsaid storage means includes a first gate between said analog-to-digitalconverter means and said storage means for transferring the codedsignals to said shift register during the range sweeps defined by thelast fill pulses, in which said storage means is a recirculating shiftregister and in which said storage means also includes a second gatecoupled between said shift register and said decoding means for applyingsignals representative of the stored codes during the range sweepsdefined by the operational pulses.
 10. A radar system havingtransmitting and receiving means comprising a first channel coupled tosaid receiving means for receiving an IF signal, attenuation meanscoupled in said first channel, moving target indicator means coupled insaid first channel, a second channel coupled to said receiving means forreceiving said IF signal, sequential detector means included in saidsecond channel, encoding means included in said second channel andcoupled to said sequential detector means, and storage means included insaid second channel and coupled to said encoding means for storing codedsignals during a selected range sweep and coupled to said attenuationmeans for controlling the IF signal in said first channel during aselected plurality of range sweeps.
 11. In a radar system of the typeincluding a radar receiver channel responsive to an input radar signalhaving a predetermined dynamic range, said radar receiver channel beingcharacterized by a limited dynamic range, smaller than the predetermineddynamic range of said input radar signal, an arrangement comprisinglogarithmic means responsive to said input radar signal for providing anoutput signal which is a function of the unattenuated input radarsignal, said logarithmic means having a dynamic range including saidpredetermined dynamic range, first means coupled to said logarithmicmeans for storing any one of a plurality of codes as a function of theamplitude of said output signal, attenuation means in said receiverchannel for selectively attenuating the input radar signal in saidreceiver channel, and second means coupled to said first means and tosaid attenuation means in said receiver channel for controlling saidattenuation means to attenuate the input radar signal in said receiverchannel as a function of said codes.
 12. The system of claim 11 whereinsaid first means include means for storing a first code when said outputsignal is below a preselected level and for storing N other codes whenthe output signal level is in any one of N different level intervalsabove said preselected level, and said second means inhibit saidattenuation means from attenuating said input radar signal in saidreceiver channel in response to said first code, and control saidattenuation means to attenuate tne input radar signal in said receiverchannel by fixed different factors in response to said N codes,respectively.